1. Field of the Invention
The present invention relates to a superwide-angle lens system, and in particular relates to a rear focusing large-aperture superwide-angle lens system that is suitable for use in an electronic still camera, in which a small solid-state image sensor is installed.
2. Description of Related Art
In the related art, a rear focusing superwide-angle lens system has been proposed in which the lens group that is provided closest to the image side is moved in the optical axis direction when focusing on an object at infinity to an object at a finite distance. For example, Japanese Unexamined Patent Publication Nos. 2004-226740 and 2008-151949 each disclose a superwide-angle lens system which focuses on an object at infinity to an object at a finite distance by moving a positive lens group that is provided at the rear end of the optical system in the optical axis direction. A rear focusing lens system enables a rapid focusing operation due to the focusing lens group (movable lens group) having a small effective diameter and being light-weight.
On the other hand, miniature electronic still cameras, in which an image sensor that is sufficiently smaller than a conventional 35 mm or APS frame size, have been developed. In recent years, higher specifications, especially in regard to a larger aperture diameter, have been in demand in optical systems used in such miniature electronic still cameras. Increasing the aperture diameter of an optical system is equivalent to increasing the effective aperture diameter of the lens groups, through which the axial light bundle passes. However, if the diameter and the weight of the focusing lens group increase, the burden on the autofocus drive mechanism increases. Furthermore, it also becomes difficult to attain a high imaging quality.
Such a superwide-angle lens system used in a miniature electronic still camera can be achieved by scaling down the optical system of the superwide-angle lens system that is suitable for a conventional large-sized image sensor such as those disclosed in Japanese Unexamined Patent Publication Nos. 2004-226740 and 2008-151949. However, these optical systems both use four or more lens elements in the focusing lens group, so that the weight of the focusing lens group is large. Furthermore, the effective diameter of the focusing lens group is too large to serve as a lens group for use in the above-described miniature camera. Whereas, if attempts are made to reduce the number of lens elements of the focusing lens group in order to reduce the burden on the autofocus drive mechanism and achieve further miniaturization of the lens system, the amount of movement of the focusing lens group during focusing increases, and it becomes difficult to achieve a high imaging quality.